PECVD provides a well known method of depositing a thin film. A body is placed in a vacuum reaction chamber and a reaction gas is introduced into the chamber. The gas is activated by means of a plasma discharge created in the chamber. This causes the reaction gas to react and deposit a thin film of a material on the surface of the body.
The methods of creating a plasma in the reaction chamber for PECVD include the method in which an electric power source having a frequency of 13.56 MHz or other frequency is applied to a pair of opposed electrodes within the reaction chamber. The deposition rate and the quality of the deposited thin film can be controlled by adjusting the power of this electric power source. Another method of creating a plasma in a reaction chamber uses a microwave radiation of 1.54 GHz, introduced into the reaction chamber by means of a wave guide. This method is known as ECR plasma CVD. Gases used to deposit a thin film of silicon oxide on a semiconductor substrate include alkoxy silicates such as tetraethylorthosilicate, (C.sub.2 H.sub.5 O).sub.4 --Si, (hereinafter TEOS) and silane, SiH.sub.4.
With the recent development of high density semiconductor integrated circuit devices (VLSI devices) there has been created an urgent need for techniques that can create ultrafine configurations in the submicron range. In order to respond to this demand, the possibility of using conventional techniques to create submicron configurations was considered by conducting an empirical study on the configuration of thin films produced by conventional plasma-enhanced CVD methods.
Each of FIGS. 1A to 1F, 2A to 2F and 4A to 4F are cross sectional views of tracings made of the outlines shown in microphotographs of the longitudinal configurations of actual devices, with the height and width of the devices being indicated by the unit scale in the figures.
Referring to FIGS. 1A to 1F, there are shown sectional views of semiconductor devices 10a to 10f, each comprising a substrate 12a to 12f having a layer 14a to 14f of an insulating material, such as silicon oxide, on a surface 16a to 16f thereof. A plurality of spaced, parallel lines 18a to 18f of a conductive material, such as aluminum, are on the insulating layers 14a to 14f, and are in turn coated with a layer 20a to 20f of an insulating material, such as silicon oxide. The conductive strips 18a to 18f have different widths, strip 18a being the widest and strip 18f being the narrowest. In addition, the spacing between the conductive strips 18a to 18f varies as well, the strips 18a being spaced apart the greatest distance and the strips 18f being spaced apart the closest distance. The insulating coatings 20a to 20f were formed by conventional PECVD wherein a reaction gas of silane (50 sccm) and oxygen at a flow rate one-tenth that of silane was passed into a reaction chamber held at a pressure of 3 Torr. A single 13.56 MHz frequency electric power source between a pair of opposing electrodes spaced 180 mils apart in the chamber was used to form a plasma between the electrodes.
As can be seen in FIGS. 1A to 1F, the sidewalls of the spaces in the silicon oxide coatings 20a to 20f have rounded edges and are thicker over the aluminum strips 18a to 18f than between the aluminum strips 18a to 18f. Thus variously shaped gaps are formed in the silicon oxide coatings 20a to 20f. As the width of the aluminum strips 18a to 18f become narrower, and the spacing between the aluminum strips 18a to 18f also become narrower, irregularly shaped sidewalls are formed and the gaps in the silicon oxide coating 20a to 20f vary in width. In particular, the gaps are narrower at the center of the sidewalls than at the bottom of the sidewalls. This leads to the formation of voids in the coatings 20 as the gaps are filled in. It is believed that because silane is very reactive, the oxidation reaction occurs in the gaseous phase, producing the non-uniform, poor deposition profiles seen in FIGS. 1A to 1F. Thus the use of silane as the reaction gas for deposition of silicon oxide films over conductive metal lines has severe limitations as devices on a semiconductor substrate become smaller and more devices are produced on a single substrate.
FIGS. 2A to 2F illustrate a cross sectional view of semiconductor devices 22a to 22f, similar to those of the semiconductor devices 10a to 10f of FIGS. 1A to 1F. The semiconductor devices 22a to 22f comprise a substrate 24a to 24f of a semiconductor material, such as silicon, having a layer 26a to 26f of an insulating material thereon, such as silicon oxide, on a surface 28a to 28f thereof. A plurality of spaced parallel strips 30a to 30f of a conductive material, such as aluminum, are deposited on the insulating layers 26a to 26f. The conductive strips 30a to 30f are in turn coated with a layer 32a to 32f of an insulating material such as silicon oxide. The conductive strips 30a to 30f have different widths, the conductive strip 30a being the widest, and 30f being the narrowest. Also the spacing between the conductive strips 30a to 30f varies, the spacing between the conductive strips 30a being the widest and the spacing between the conductive strips 30f being the narrowest.
In FIGS. 2A to 2F, the insulating layers 32a to 32f were formed by PECVD using TEOS as the reactive gas, and a single 13.56 MHz power source. As can be seen from FIGS. 2A to 2F, the sidewalls of the silicon oxide deposit are less rounded than those shown in Fig, although the gaps between the openings in the silicon oxide layers 32a to 32f narrow as the conductive strips 30a to 30f become narrower and the spacing between them becomes narrower. Thus the use of TEOS to form silicon oxide films has advantages over the use of silane for small features. However, these films finally also form voids, particularly when the spacing between the strips 30a to 30f is less than 0.5 micron, see layers 32d to 32f. Thus although the deposition profile is much more uniform that the profiles of FIGS. 1A to 1F using silane as the precursor gas, as the aluminum lines become narrower and closer together, voids are formed in the growing film. Thus the use of TEOS to grow silicon oxide films over stepped topography is also limited, and is inadequate for submicron lines and spaces as shown particularly in FIGS. 2D to 2F.
Thus it would be desirable to be able to deposit silicon oxide films over conductive lines that are spaced closer than 0.5 micron, but that do not form voids in the layer. Further, it is desired to improve the quality of the silicon oxide films.
Other prior art workers have addressed this problem. Foster et al, U.S. Pat. No. 4,667,365 and assigned to the same assignee as the present invention, discloses adding CF.sub.4 or NF.sub.3 to silane while depositing silicon oxide, and to creating the plasma using either a single power source or more than one power source having different frequencies, and using magnetron enhanced processing. Foster et al explain that the fluorine-containing gases produce etching during deposition, thereby controlling the sidewall profiles and improving the conformality of the depositing film and reducing the formation of voids. However, the deposition rate was adversely affected by the addition of the fluorine-containing gases.
Another process that has been used to improve the ability of the SiO.sub.2 film to adequately fill the gaps between closely spaced apart metal lines comprises an atmospheric pressure (i.e., 760 Torr) CVD method in which TEOS gas is utilized with a source of oxygen gas comprising a mixture of O.sub.2 and O.sub.3 gases, i.e., an ozone-TEOS precursor gas mixture. This method however, leads to the formation of a silicon oxide film having a high hygroscopicity, which can lead to degradation in the moisture resistance of the final integrated circuit structure, which is not tolerable.
Weise et al, PCT application US92/04103, describes the reaction on an inorganic substrate of unsubstituted silane (SiH.sub.4) together with a halogen-containing gas and an oxygen-containing gas by PECVD or ECR CVD techniques. Alternatively the precursor gas can be an organosilane. An etchant is added along with the precursor gas or gases. Suitable etchants listed include fluorine-containing compounds and halogens, but the preferred etchants are HF or NF.sub.3. Sulfur-based or carbon-based etchants are not preferred however, because it is stated that residual sulfur or carbon remains in the films, which is undesirable. Halogens are not preferred either, because they corrode the reaction chamber and other equipment. As is well known, NF.sub.3 and HF are also corrosive, particularly to quartz parts. The addition of NF.sub.3 to the silicon oxide film reduces intrinsic stress in the film, and also reduces the amount of hydrogen present in the film, which has a high dielectric constant. However, this process leads to films having low compressive stress, which leads to semiconductor devices with unsatisfactory electrical properties, and inferior mechanical properties. The process also exhibits low deposition rates.
Thus a method of depositing silicon oxide films on semiconductor substrates, and particularly over submicron conductive metal lines, that is conformal even over submicron lines and spaces, and that is of high quality, but without sacrificing deposition rate unduly, would be highly desirable.